Coal is a solid, combustible mineral substance which, as a result of its natural coalification process, is generally associated with some non-combustible mineral matter, called ash. In conventional coal cleaning processes, such as coarse or intermediate gravity preparation, removal of the larger fragments of the inert material is highly successful whereas removal of the finer fragments intimately associated with the coal is largely inefficient.
Fine cleaning of the -28 Tyler mesh coal is currently accomplished by a process which integrates classification, cyclone washing and flotation. With the ever increasing concern over environmental problems associated with "black water" and the increase in the value of coal as an alternative energy source, the recovery of the fine coal through flotation techniques is becoming more prevalent in the industry as a whole.
It has long been known that coal possesses an inherent flotability. In the froth flotation of coal, coal is separated from its gangue in the form of a froth using gas as the buoyant medium. Since coal is flotable, the material generally may need only a nudge to float readily. Thus, the high rank bituminous coals frequently require only a frother. However, the intermediate to low rank bituminous coals may require from 1 to 3 pounds of hydrocarbon oil per ton of coal for good recoveries. Lignite further requires even larger dosages of hydrocarbon oil or more potent collectors to achieve good recovery. With the oxidized coals, large quantities of hydrocarbon oil and/or collectors are again necessitated to obtain the desired recoveries.
In an effort to increase recovery of the fine coal without increasing the amount of ash in the recovered product, various novel polymers have been suggested in the art. Frank F. Aplan reported in his article "Coal Flotation", Flotation, A. M. Gaudin Memorial Volume, Volume 2, The American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., New York, 1976, that emulsifying a fuel oil with a frother or a surfactant can lead to reduced reagent requirements in froth flotation of coal. M. Barcal and F. Dedek reported in "Emulsification of Flotation Agents for Coal", Acta Mont. 1974, No. 28, 59-91 that improved floatability was observed with a stabilized emulsion of a mineral oil through the addition of a foaming agent. Although both of these references teach the use of oil-in-water emulsion systems, they indicate that the recoveries achieved still lag behind industry expectations. More recently, U.S. Pat. No. 4,162,966, issued to Finch in July of 1979, disclosed that the employment of water-in-oil emulsions of a sodium polyacrylate latex as a flotation promotor for coal increased recoveries. The leading edge of innovation in this subject matter, therefore, requires employing, as a flotation promoter, a water-in-oil emulsion comprising a polymer latex, a paraffinic solvent, an emulsifier, an activator and a minor quantity of a stabilizer. The Finch emulsion promotor is not only commercially unattractive due to the high cost of the polymer, but has been shown to be indiscriminate and non-selective in its recovery strength resulting in a high ash recovery with the coal.
Accordingly, there exists the need for a more efficient and economic promoter capable of exhibiting high coal recoveries without associated ash. Accordingly, the provision for a more economical and efficient promoter would fulfill a long-felt need and constitute a major advance in the art.